Add returnUrlVerifier tests
Add returnUrl to sign in form

This is done using IdentityUrlBuilder on the form's action. This class
contains a buildUrl method that correctly appends Identity-specific
querystring parameters such as returnUrl.

IdentityUrkBuilder in turn depends on an updated IdentityRequest and
the returnUrlVerifier now deals with Options to make this work nicely
across the application.
Update Identity properties for other envs
Merge remote-tracking branch 'origin/master' into signin-form-with-id-api
Simplify URL builder by requiring leading / on path
Merge pull request #1372 from guardian/signin-form-fixes

ID Signin form fixes